1. Field of the Invention
The present invention relates to an electromagnetic coupling device used, for instance, for feeding papers in a copy machine. More particularly, it relates to an electromagnetic coupling device used under condition of a reduced load such as a transmission torque of about 5 kg.cm and number of revolution at the input side of about 200 rpm.
2. Description of the Prior Art
FIG. 1 shows a conventional electromagnetic coupling device. In FIG. 1, a reference numeral 1 designates a stator in a form of a yoke holding therein a coil 2. The stator 1 is immovably engaged with a stationary member (not shown). A rotor 3 which is a first coupling body fixedly securing a lining 4 is attached by forcibly fitting to a hollowed rotary shaft 5 made of magnetic substance such as iron. A hub 6 provided with a gear wheel 7, which is usually formed by molding plastic material or by sintering powder of an alloy, is directly attached to the rotary shaft 5 so as to be able to relatively rotate. A returning leaf spring 8 is secured to the hub 6. An armature 9 as a second coupling body is fixed to the leaf spring 8 by a rivet 10 at a position opposing the lining 4. A predetermined gap g is formed between the armature 9 and the rotor 3. A reference numeral 11 designates an iron series metal inserted between the stator 1 and the rotary shaft 5, a numeral 12 designates a spacer, numerals 13, 14 designates retaining rings fitted to the rotary shaft 5 to fix positions of the stator 1, the rotor 3 and the hub 6 in the axial directions.
In the conventional electromagnetic clutch constructed as above-mentioned, when a current is supplied to the exciting coil 2 while the gear wheel 7 is driven by a motor (not shown), a magnetic flux .phi. is produced to form a magnetic circuit in elements constituting the electromagnetic clutch as shown in FIG. 1, whereby the armature 9 is attracted to the rotor 3 against the spring action of the leaf spring 8. Rotation at the driving side is transmitted to the rotary shaft 5 through the rotor 3 via the hub 6, the leaf spring 8, the armature 9 and the lining 4. The rotary shaft 5 transmits a rotational force to a driven shaft 16 which is fitted to the hollowed rotary shaft 5. When supply of a current is stopped, the magnetic flux .phi. disappears and the armature 9 is separated from the rotor 3 by the spring action of the leaf spring 8; thus the rotary shaft 5 is stopped.
In the conventional electromagnetic clutch, since the rotary shaft 5 contributes to formation of the magnetic circuit, it is necessary to use soft iron having an excellent magnetizing characteristic. Accordingly, it is necessary to interpose a metal 11 between the rotary shaft 5 and the stator 1 which also contributes formation of the magnetic circuit since it is difficult to made the rotary shaft and stator to be in a direct slide-contacting state. Insertion of the metal 11 provides such disadvantages that frictional resistance at the sliding portion becomes large; a magnetic attractive force of the magnetic circuit produces a resistance of rotation between the stator 1 and the metal 11 thereby resulting in relative rotation between the stator and the metal; and abrasion caused in the sliding portion does not impart a sufficient performance for a long term.
Since the metal 11 also contributes formation of a magnetic circuit, it is difficult to use material other than an iron series metal (including one coated with a sliding agent). Further, the rotary shaft 5 made of soft iron becomes rusty whereby faulty sliding movement may take place. For rustproof effect, zinc plating is sometimes employed as a method of treatment of surface. However, it is not satisfactorily reliable since hardness of zinc is low. It is also considered to use nickel plating, chromium plating and so on. However, this expedient has the drawback of being expensive. In addition, machining operations of the rotary shaft 5 are not easy because the rotary shaft is made of soft iron. Two grooves for retaining rings have to be formed in the rotary shaft 5 since the retaining rings 13, 14 are required to fix the stator 1, the rotor 3 and the hub 6 in the axial directions. Accordingly, efficiency in manufacturing the electromagnetic coupling device is lowered.